Note: Descriptions are shown in the official language in which they were submitted.
" 1080~1;2
The invention concerns a blue-flame oil burner, that is an
oil burner intended to operate with a blue flame and having
provision for recirculation of part of the combustion gases and
having an oil-atomising device, a wall containing a metering
orifice arranged downstream of the outlet of the oil-atom~ing
device, a mixing tube arranged at a distance downstream of the
orifice and coaxial with it and a flame-tube around the mixing
tube.
Blue-flame oil burners require that the oil reaching the
point of combustion is completely vaporised before it reaches
that point. The operation of an oil burner with a blue flame
has the advantage that the burner is able to operate with very
small excess of air over that required for complete combustion
so that practically stoichiometric combustion takes place.
Since combustion takes place with very small excess of air a
very hot flame is produced which utilises the energy content
the fuel optimally and leads to improved heat transfer. In
addition, the waste gases in comparison with waste gases from
an optimally adjusted burner with a yellow flame contain
extremely little harmful material (soot, NOX, SO3).
Because of the known advantages of an oil burner in which
oil is burnt in the vaporised state with a blue flame, there
have been no lack of attempts to bring oil burners with blue
flames on to the market.
In a known blue-flame type of oil burner an orifice plate
is arranged in a double-walled combustion chamber downstream of
the ~il nozzle. Oil is sprayed through the orifice plate into
an ~long~te mixing tube and, with the aid of air which enters
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the tube simultaneously through the orifice, partial combustion
takes place with an excess of combustible material. These -
combustion gases are returned and again enter the end of the mix-
ing or vaporisation tube opposite the orifice plate at the same
time as the spray of fuel. In this known burner the inner wall
is provided with holes through which the greater part of the air
of combustion enters in the form of a secondary air flow. (Proc.
World Petr. Congr., 7th Volume 7, Sect. on Application and New
Uses, Part 1, p 119 f f ) . `
An oil burner with a blue f lame with recirculation is
also known in which the orifice plate is situated in a tube which
exte~ds so far in the upstream direction that the nozzle and the
nozzle support are situated completely within the tube. With
such a burner the flame should burn immediately behind the ori-
f ice plate and part of the combustion gas should be recirculated
through an annular space between the tube and an outer tube
(ibid).
An oil burner is also known in which oil is sprayed
directly into a tuhe which is surrounded by an outer recircula-
tion tube connected to the inner tube by means of bores UIC Pat-
ent F18213 issued on November 10th 1954 to Thermal Research and
Engineering Company.
It is an object of the invention to design a blue-
flame oil burner incorporated features of ,~r~nced conventional
technology and which has a high operating reliability.
According to the invention there is provided an oil
burner of the kindin which vaporised fuel is capab1e of burning
with a blue flame and having provision for recirculation of part
of the combustlon gases within a flame tube, the oil burner com-
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prising the flame tube, a wall extending transversely of the
flame tube and defining the upstream end thereof, the wall having
therein a metering orifice through which air enters the flame
tube, the orifice being the only air inlet into the flame tube,
an oil atomising device positioned upstream of the wall and arran- ~
ged to discharge an oil spray through the orifice into the flame :
tube, and a mixing tube positioned within the flame tube and ~
aligned with the orifice, the mixing tube having its upstream end ~.
spaced axially from the wall by a distance such that the periph-
eral area of the space between the wall and the upstream end of
the mxing tuhe and defined within an imaginary upstream extension :~
of the peripheral wall of the mixing tube to the transverse wall
is between one and three times the difference in cross-sectional
area between the upstream end of the mixing tube and the orifice,
the mixing tube having a length, L,and a cross-sectional area at
the upstream end thereof equal to the area of a circle having a `~
diameter, D, such that the ratio L/D is between 1.0 and 1.75,
and the flame tube having an equivalent ratio L/D of between 2.0
and 5Ø
. Preferably the ratio L/D of the mixing tube is equal
; 20 to or is approximately 1.5.
Preferably also the ratio L/D of the flame tube is
between 2.5 and 3Ø
In the preferred embodiment the cross-sectional area
of the mixing tube is between 1.5 and 3.0 times the cross-sect-
ional area of the orifice.
The flame tube and the mixing tube may have constant
cross-section over their length. For example, the flame tube and
the
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10801~Z
mixing tube may both be designed to be cylindrical.
It may,of course, be convenient for silencing to design the
flame tube wlth a cross-section of varylng orm, ~or ex~mple
with a conLcal enlargement or even in a star shape.
A special problem with blue-flame oil burners ls the
reliable ignition of the flame in the variable starting
conditions which occur in practice, for example relighting a
burner which is still hot. According to a feature of the
invention the necessary reliability of ignition is achieved in
that the burner is started with an air pressure below that of
the full-load operation. The supply of air is then quickly
increased after the opening of the oil valve to an air fraction
sli~htly above the stoichiometric ratio with which the oil
burner is then supplied.
Operation in the manner described is preferably achieved
in that the air flap of the blower is provided with a drive and
that time switch means are provided for operation of the drive.
For example, an electric or hydraulic motor may be provided as
the drive which is preferably connected to the air flap
through a self-locking gear mechanism.
It is also possible to use an electrically excited magnetic
device or a hydraulic cylinder in each case with time-controlling
damping members.
It may also be convenient when starting-up either in
conjunction with or instead of throttling the air supply to the
burner to operate with a throttled flow of oil mixture which
can then be quickly brought up to the full air-oil mixture
flow.
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By way of example an embodiment of the invention is
illustrated in the accompanying drawing and is described in
detail in the following with reference to the drawing, which
shows a longitudinal section through an oil burner according
to the invention together with its control and supply devices.
The burner 2 illustrated includes a chamber 4 in which a
pressure atomising nozzle 6 is mounted in a known manner at
the end of an oil supply pipe 8. Oil is fed into the pipe 8
by an oil pump 10 which is driven by a motor 12 which also
drives a blower rotor 14 in a known manner. The pump 10
delivers oil through a hand-operated throttle valve 16 and an
electromagnetically-operated shut-off valve 18 into the oil
pipe 8 which carries the atomising nozzle 6. The blower 14
delivers air through a duct 20 into the housing 4 through a
lS throttle valve 22 having air flap 24 whlch can be adjusted by
a motor 26. A pair of ignition electrodes 30 is carried by a
support 28 mounted on the oil pipe 8 and is connected to an
ignition transformer 32.
At a distance L3 from the mouth of the pressure atomising
nozzle 6 there i8 a wall 34 having a metering orifice 36
therein coaxial with the nozzle. At a distance from the
orifice 36 there is a mixing tube 38 which is attached to the
wall 34 by mounting rods 40. The mixing tube 38 is situated
inside an outer tube 42 which forms the flame tube. In this
example, the orifice 36, the mixing tube 38 and the flame-
tube 4Z are circular but they need not be.
The diameter D3 of the orifice 36 and the arrangement and
dimensions of the mixing tube 38 and of the flame-tube 42 are
in a prdeter~ined critical relationship to each other which
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will be discussed in detail in the following.
The atomising nozzle 6 should be adjustable in the
axial direction in order to vary the through-put of air and
the oil cloud input and thus make it possible to achieve the
optimum composition of the mixture.
The diameter D3 Of the orifice 36 should be chosen in
such a way that at a delivery pressure of the blast 14 which
corresponds to the delivery pressure normal in commercial oil ~;
burners, the air flow through the orifice 36 in the wall 34
has a predetermined velocity, the orifice being the only pas- - r
sage for combustion air.
'l The diameter Dl of the mixing tube 38 is at least 1.25
and at maximum 1.7 and is preferably 1.4 to 1.45, times the
diameter D3 of the orifice 36. Alternatively, the ratio of
the cross-sectional area of the mixing tube and the orifice is
between 1.5 and 3.0 where the mixing tube and/or the orifice
are not of circular cross-section. The length Ll of the mixing
tube is between 1.75 and l.ODl where Dl is the diameter of
the mixing tube, and preferably Ll = 1.5Dl.
The distance L4 of the mixing tube 38 from the wall 34
is chosen in such a way that a flow cross-section, radially of
the mixing tube 38, which is at least 1 to 3 times the differ-
ence between the cross-sections of the orifice 36 and of the
mixing tube 38 is produced between the upstream end of the mix-
ing tube 38 and the wall 34. In other words, the peripheral
area of the space between the wall 34 and the upstream end of
the mixing tube 38 and defined within an imaginary upstream
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extension of the peripheral wall of the mixing tube 38 to the
wall 34 is between one and three times the difference in
cross-sectional area between the upstream end of the mixing
tube 38 and the orifice 36. The diameter D2 of the flame tube
42 is approximately twice to 2.5 times the diameter Dl of the
mixing tube and the ratio of the length L2 to the diameter D2
of the flame tube is between 2:1 and 5:1 and is preferably
between 2.5:1 and 3:1.
A spray cone of approximately 60 to 80 degrees is
convenient for the pressure atomising nozzle 6. Particularly
good results were obtained with nozzles with hollow spray
cones. When the oil burner is to be started, the motor 12
is first switched on inthe normal way. At the same time, the
throttle flap 24 is closed unless it had already been closed
during shutting off of the oil burner. A short time later
voltage is applied to the ignition electrodes 30 and the
; electro-magnetically operable valve 18 is subsequently opened
so that oil supplied by the oil pump 10 reaches the pressure
atomising nozzle 6. At the same time, air is supplied by means
of the blast 14 to the chamber 4. The mixture produced is
ignited by the ignition electrodes 30. A flame is thereby
produced in that part of the flame tube 42 situated in front
of the downstream end of the mixing tube 38. At the same time
that ignition occurs, the throttle flap 24 is moved into the
operating position under the control of control apparatus.
The diameter of the orifice 36 is such that the air inside the
mixing tube 38 has a velocity such that drops of atomised oil
do not form on the inner wall of the mixing tube 38 when the
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air throttle flap is open. By means of the core stream of
air, oxygen-poor combustion gas is sucked through the annular
space around the mixing tube 38 and through the upstream end
of the mixing tube 38 or through openings in the rear wall of
the mixing tube at its upstream end. These hot combustion
gases surround the core stream and give up heat thereto. An
additional criterion for the dimension of the orifice 36 and
the air velocity is that the air velocity in the mixing tube
! 38 should be greater than the velocity of propagation of the
flame in order to ensure that the flame burns at a distance
from the downstream end of the mixing tube 38. The stated
operating conditions give the result that the distance between
the end of the mixing tube 38 adjacent the wall 34 and said
wall 34 is small as possible. This distance must be chosen
in such a way that the recirculating gases flowing into the
mixing tube 38 are as loss-free as possible and are able to
mix with the oil cloud/air mixture of the core stream for the
purpose of heat exchange. With experlmental burners the
following dimensions have been found to be favourable:-
1. Using a pressure atomising nozzle giving a throughput
of 0.5 to 0.65 gal/hr and
Diameter of the orifice D3 23.5-26 mm
Internal diameter of the mixing tube Dl 35 mm
Internal diameter of the flame-tube D2 76 mm
a very stable blue flame was produced over the whole
range of throughput.
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2. Using a pressure atomising nozzle giving a throughput
of 0.65 to 0.9 ga~/hr, the *bmeter D3 of the orifice
36 was increased to 25-31 mm with the remaining
dimensions kept constant. Once again a stable blue
flame was produced over the whole range of throughput.
A series of models developed for fabrication had the
following principal dimensions (all meansurements in mm):-
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Model Dl D2 D3 Ll L2~ L4 60Zle
1- gal/hr
Li~
2 36 77 26 S7 225 9 0.65
3 36 77 29 52 225 14 0.75
In all the models the distance L3 of the nozzle opening from the
front face of the wall 34 was 2 mm.
Maintenance of the following combustion data was established
for all ~odels on the test bench:-
C2 = 155~ (approximate theoretical maximum)
Soot = 0
CO = 0.01
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With blue flames, monitoring of the flame cannot be carried out
optically. To guarantee a reliable automatic operation of the
blue-burning flame monitoring is possible by means of an
; ionisation detector 44 which is connected in known manner to r
a control device 46 by means of which, when tlie flame is
extinguished, the supply of oil is cut off by closing the
valve 18 and the motor 12 is switched off. After the flame
has been produced the ignition device is also switched off by
the control device in known manner.
It has been shown in practice that it is essential that
the mixing tube 38 should be designed with as small a volume
as possible, and thus, for given dimensions, with as small a
wall thickness as possible. This ensures that the mixing tube
38 will glow bright red, so that a large portion of the heat
of the recirculating combustion gases is transferred to the
mixture of air and oil drops in the form of radiant heat, so
that, once again, the complete vaporisation of the oil without
droplets impinging on hot surfaces is ensured. With the
aforesaid ratio Ll:Dl for the mixing tube it is ensured that
the mixing tube is surrounded over its whole length by oxygen-
poor combustion gases so that with mixing tubes of heat
resistant steel it is not possible for any marked scaling or
oxidation of the mixing tube to occur. The mixing tube may
also be made from heat resistant ceramic material. The
support of the mixing tube 38 can be effected, instead of by
the axial arms 40, by radial arms which should then be mounted
on the inside of the flame tube 42.
With stoichiometric combustion in the flame tube 42
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1080~12
practically no free oxygen is present. This is another
reason why the flame tube 42 may be made of heat resistant
steel without risk of wear due to scaling or oxidation.
A~ernatively, the flame tube 42 may be made from a heat
resistant ceramic material or a steel tube having a heat
resistant ceramic coating may be used. It is possible to
arrange for the flame tube to be cooled, for example, the
heating water. In this case the flame tube may, for example,
form part of the heat exchange system of the boiler. With
cooled flame tubes, it is not necessary to use hightly heat
resistant materials.
Combustion in the oil burner described is to a large
extent independent of the size and shape of the combustion
chamber of the boiler. Under very unfavourable conditions,
resonance phenomena may, however, occur during the ignition
phase. These can be prevented by providing the wall of the
flame tube 42 with a few holes in the region of the flame
front. The development of noise may also be caused by a change
in the flame cross-section. For example to reduce noi~e, a
conlcal expansion of the flame-tube may be provided or the
flame-tube, downsteeam of the flame zone, may be given a star-
shaped cross-section.
A special problem with oil burners burning with a blue
flame is that of guaranteeing reliable ignition under all
working conditions. According to a feature of the invention,
reliable ignition is achieved by, at the moment of ignition,
supplying the burner with less than the stoichiometric air
quantity required relative to the nominal load. For this
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purpose the air flap 24 of the throttle valve 22 is moved to
a suitable starting position by the adjusting motor so that
on fresh ignition only, a suitably adjusted less than
stoichiometric air quantity is supplied to the chamber 4. After
the oil has ignited, the air flap 24 is moved by the adjusting
motor 26 into the open position in which the quantity of air
required for stoichiometric combustion is allowed to pass
through the valve 22. The adjusting motor 26 may be switched
on at the same time that the electro-magetically-operable
valve 18 is opened, a finite time delay necessary being
produced by means of a step-down gearing and, if necessary,
accessory electrical control elements. This method of
operation has the advantage that the air quantity is increased
continuously. Switching on of the motor may also occur in
dependence of the detection of a flame by means of the
ionisation indicator 44. As the drive for the air flap, it
would also be possible to provide an electro-magnetic device
with known means of delay. A delay could be ensured for both
an electro-magnetic device and an adjusting motor by means of
a semi-conducting resistor. It has been established that,
in general, in order to achieve reliable ignition, it is
sufficient to start the burner with less than stoichiometric
air quantity for a duration of about 3 to 5 seconds. It has
been established that within this time, a stable recirculation
flow is achieved with simultaneous heating of the mixing tube
to the operating temperature. During starting with less than
the stoichiometric air quantity, as described, no formation of
soot was observed. Probably the .eason for this is that the
amount of air present at any time within the combustion chamber
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of the boiler is sufficient to ensure that the necessary
oxygen for complete combustion is available. Depending on the
conditions in the boiler at a particular time it may be necessary
to increase the time allowed before the complete opening of the
throttle flap occurs to 6 to lO seconds.
From a general point of view it is important for reliable
ignition, to start the burner with an air velocity less than
the air velocity at full load. Conveniently, it has been shown
that during starting an excess pressure of 23 to 25 mm head of
water during full air-fuel mixture flow.
If a hydraulically-operated adjusting motor is used, it is
possible to employ the heating oil compressed by the pump 10
as the pressure fluid. Under certain circumstances it may
also be convenient to provide ~ans by which it is possible to
start the burner with, in addition to the throttling of the
air stream, a throttled stream of oil as well. Then the stream
of oil is raised to full flow in the same time period as that
stated hereinbefore for the air stream. It is possible in
this case, to keep the period of the less than stoichiometric
operation short during starting and in the ideal case to
maintain stoichiometric conditions of combustion even during
starting the burner. It is, a necessary condition that even
with a decreased flow of oil,the atomisation of the oil is
not impaired. Up to a certain size, larger oil drops are
acceptable since the walls, in particular the wall of the mixing
tube, the temperature of which is essentially equal to the
temperature of the boiler water (about 70-90C) during the
starting process, have a cer~n storage capacity, that is to
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say larger drops of oil can settle on the walls at first and
form an oil film there which will evaporate once full air-
fuel mixture flow has been achieved.
In order to be able to operate the burner with maximum
air-fuel mixture flow under varying conditions with as constant
an excess of air as possible, that is as accurately
stoichiometrically as possible, it is convenient to provide
means by which it is possible to adjust the flow rates of
air and/or oil automatically in response to momentary changes
in operational conditions, in particular air pressure, air
tempeature and/or the boilder draught.
Withp~essure atomising nozzles having a high throughput,
the fine atomisation necessary for vaporisation is often no
longer achievable. For burners of high power it may
therefore be convenient to arrange a plurality (at least two)
of nozzles at a distance from one another, either parallel or
at an angle to each other, where the cross-section of the
orifice 36 in the wall 34, the mixing tube 38 and, where
necessary, the flame-tube 42 are geometrically adapted in a
suitable manner.
The orifice 36 may, as illustrated, be a simple stamped-
out opening in a disc-shaped wall 34. The orifice 36 may,
however, be formed so as to project for a short distance in
the form of a tube with an angular or a rounded-off transition
to the wall 34.
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